The invention described herein relates to a bioherbicide utilizing the fungus Myrothecium verrucaria for controlling Salvinia molesta Mitchell (SAMOS). Salvinia molesta is a floating tropical fern that is native to Brazil. Salvinia molesta—commonly known as giant salvinia—has been described as one of the two worst aquatic weeds in the world, along with water hyacinth. In the tropical and subtropical regions of the world where infestations occur, the impact of giant salvinia on human activities can be devastating. Because the plant is often introduced without its natural enemies, giant salvinia often becomes invasive, replaces native flora, and disrupts ecosystems. In addition to the United States, the plant has been reported in more than 20 countries.
A typical giant salvinia plant is comprised of units (ramets) of three leaves (fronds). Ramets are joined by underwater stems (rhizomes) that have apical and axillary buds. The two floating leaves are covered with numerous leaf hairs, which make these leaves resistant to wetting. Giant salvinia can be identified by the cage-like configuration of the leaf hairs. Each submerged leaf is finely dissected and these are often mistaken for roots. These modified leaves, and associated sporocarps, trail beneath the surface of the water and can be several feet in length.
Three growth stages have been identified: primary, secondary, and tertiary. Population densities and nutrient levels influence these growth patterns. At low populations densities, the plants remain in the primary stage of growth; however, as plants become more crowded, the growth pattern changes to secondary, and then to tertiary stages of growth. These growth patterns are well documented and have been described in detail elsewhere [See e.g., Van Oosterhout, E., Salvinia control manual: Management and control options for salvinia (Salvinia molesta) in Australia, NSW Department of Primary Industries (ISBN 0 7347 1747 4) (2006)]. Under ideal conditions, giant salvinia biomass can double in 3 days and reach levels of 400 tonnes of fresh weight per hectare (178.5 short tons per acre). These growth rates can exceed the capability of control by mechanical removal. Floating mats in the tertiary stage of growth can be multilayered—sometimes 2 feet thick—essentially blocking all sunlight penetration into the water.
Myrothecium verrucaria—a cosmopolitan, soil inhabiting fungus—has previously been shown to be effective in controlling various flowering seed plants. For instance, U.S. Pat. No. 5,747,029 (Walker et al.), hereby incorporated by reference, teaches methods for the biological control of various flowering seed plants (both monocot and dicot species) including sicklepod, pigweed, spurred anoda, jimsonweed, and hemp sesbania using Myrothecium verrucaria. U.S. Pat. No. 6,274,534 (Boyette et al.), also hereby incorporated by reference, describes the use of Myrothecium verrucaria for control of kudzu, also a dicot species.
While these works teach that Myrothecium verrucaria is effective in controlling a number of dicot species of flowering seed plants, they also document that there are a number of species of monocots and dicots that are resistant or immune to the bioherbicidal activity of the fungus. The bioherbicidal activity of the fungus could not be predetermined for specific species of flowering seed plants. In fact, effective control of a given species by Myrothecium verrucaria was shown to be more the exception than the rule.
The present invention describes the use of Myrothecium verrucaria to control giant salvinia, a species of aquatic fern. Apparently, Myrothecium verrucaria has not been reported as a pathogen of giant salvinia. Moreover, ferns are taxonomically very different from the flowering seed plants in that ferns do not produce flowers, fruits, or seeds. In the six kingdom classification system, the Kingdom Plantae places ferns in the phylum Polypodiophyta, while flowering seed plants are placed in the phylum Magnoliophyta, which is divided into the classes Magnoliopsida (dicots) and Liliopsida (monocots). Because of the taxonomic and biological differences that exist between the flowering seed plants and the ferns, the bioherbicidal activity Myrothecium verrucaria toward ferns, and in particularly giant salvinia, could not have been predicted based on the prior art.